Update protobuf to 3.12
Currently Paraview uses protobuf 3.11.2. But the latest protobuf version 3.12.* is binary-incompatible with 3.11. This creates a problem (paraview crashes on start) when we dynamically link a plugin that uses protobuf 3.12.
We currently work around this problem by downgrading protobuf to 3.11.2 on our side, but it's better if Paraview can follow the newest protobuf.
If someone can tell me a way to do that, I'm happy to create a pull request. It looks like ThirdParty/protobuf is where we import protobuf, but it's not clear how we update the protobuf version.